The invention relates to frequency synthesized microwave signal generators having particular application as signal generators for radar transmitter and receiver local oscillators requiring frequency stability, spectral purity, frequency agility, wide operating bandwidth and relatively small packaged volume.
Signal generators that are used to produce tactical airborne radar transmitter and receiver local oscillator (LO) signals are unique in the severity of the requirements placed on them for frequency stability, spectral purity, frequency agility, operating bandwidth and packaged volume.
Prior techniques for generating the high quality signals needed for radar applications have generally been a compromise of optimizing some parameters at the expense of others. A common prior technique uses either a frequency comb generator or a programmable harmonic phase locked loop to provide coarse (approximately 100 MHz step size) frequency control. A separate phase locked loop is used to offset the selected coarse set frequency through a mixing operation and a selectable loop reference frequency that is provided by a bank of crystal oscillators. That scheme separates the functions of coarse and fine frequency control and their combination and provides excellent noise and spurious performance at the expense of slow, 5 millisecond frequency switching and large packaging volume. Also, the technique used to control the harmonic lock ratio is restricted in practical usage to approximately six (6) coarse frequency steps.
A second prior technique uses indirect frequency synthesis and is based on the use of wide range programmable frequency dividers in the reference and feedback signal paths of a series of phase locked loops that provide progressively finer levels of frequency control. While that approach can provide arbitrarily fine frequency steps, the noise introduced by the large ratio dividers results in degraded output FM noise performance.
A third prior technique uses direct frequency synthesis and is based on the use of a cascade or other combination of mixers, frequency multipliers and dividers, and filters to generate the desired frequencies. Although that approach can provide the fastest switching speed, it is generally limited in spurious performance and highly volume consuming.